A CLASS ACT means that something or somebody is of particularly high quality.

For Example:

Dave’s rap performance last night was a class act. He’s one of the most original rap artists on the scene.

If something has class, it also means that it is somehow better quality or “cooler”.
The new secretary’s really got class. (Meaning she does a great job, and/or she is attractive.)

The new MP3 player really is in a different class; it has more functions and a cheaper price than any other model. (Meaning it is much better than similar MP3 players).